In view of present environmental regulations, the gasoline specification for sulfur content is becoming limited to lower levels. The main source of sulfur in gasoline is catalytic cracked naphtha, which can contain typical values of 1,000 to 1,500 ppm wt. Besides the organosulfur compounds, the FCC naphtha includes typical olefin contents in the range of 25 to 35 mass %.
The conventional fixed bed hydrodesulfurization process (HDS) permits the attainment of low sulfur contents, but implies in the undesirable hydrogenation of olefins present in FCC naphtha, resulting in octane losses of the final gasoline pool containing FCC naphtha hydrodesulfurized stream.
Therefore there is a huge demand for the maintenance of the gasoline octane rating and hence, for processes aiming at reducing the sulfur content while maintaining the naphtha olefins. Several selective hydrodesulfurization technologies have been developed, where selectivity means the ability to remove sulfur with minimum olefin hydrogenation.
For example, an olefin-rich naphtha stream can initially be split into two distillation cuts, a heavy one and a light one, so that only the heavy cut undergoes a hydrodesulfurization reaction. By combining the two cuts after the reaction it is possible to keep the olefins of the light, more olefinic cut, so as to obtain a low-sulfur gasoline while preserving the octane rating.
U.S. Pat. Nos. 2,070,295, 3,957,625, and 4,397,739 describe such a process, however, a certain amount of sulfur remains in the light naphtha, so that the literature teaches processes including a further alkylation step of the thiophenic sulfur in the light naphtha so as to concentrate the sulfur in the heavy naphtha, such as described in U.S. Application 2003/0042175.
U.S. Pat. Nos. 3,957,625, 4,334,982, and 6,126,814 teach catalyst compositions where the catalyst features selectively favor the hydrodesulfurization function while reducing the olefin hydrogenation function.
Contrary to usual hydrorefining catalysts, HDS processes directed to olefinic naphtha streams employ Group VI B (MoO3 being preferred) transition metal oxides and Group VIII (CoO being preferred) transition metal oxide catalysts in sulfided form during operation conditions, supported on suitable porous solids. Preferably the acidity of the supports is diminished with the aid of additives, or the acidity is intrinsically low. Also known are variations in metal contents and optimum ratios between them so as to favor the hydrodesulfurization while the hydrogenation of the olefin function is reduced.
For example, U.S. Pat. Nos. 4,132,632 and 4,140,626 describe the selective desulfurization of cracked naphtha streams using catalysts containing specified amounts of Group VI-B and Group VIII metals on a magnesia support containing at least 70% by weight of magnesium oxide and that can also contain additional refractory inorganic oxides such as alumina, silica or silica/alumina.
On the other hand, U.S. Pat. No. 5,441,630 makes use of catalysts of the same Group VI-B and Group VIII metals supported on a mixed basic oxide resulting from the mixture of hydrotalcite and alumina. The contents practiced in the mixture of hydrotalcite and alumina is from 1 mass % to 70 mass % hydrotalcite, preferably from 20 mass % to 60 mass % hydrotalcite.
U.S. Pat. Nos. 5,340,466 and 5,459,118, of the same Applicant as the above '630 patent teach a selective desulfurization process of cracked naphtha streams using a catalyst similar to that of U.S. '630, with additional deposition of Group I-A alkaline metal (such as K2O).
U.S. Pat. No. 5,851,382 of the same Applicant teaches the use of the same metals of Group VI-B and Group VIII and added Group I-A, where the support comprises essentially hydrotalcite (above 80 mass %) and less than 20 mass % of a binder to allow extrusion. As binders are used silica, silica-alumina, titania, clays, carbon and their mixtures, but not alumina, this leading to higher selectivity towards sulfur removal with lower olefin hydrogenation as compared to catalysts of previous U.S. patents of the same Applicant containing alumina in the support composition.
Further patents directed to processes for the naphtha hydrodesulfurization claim the use of selective catalysts. U.S. Pat. No. 6,231,754 teaches the use of a catalyst rendered selective by the use of low metal contents, the catalyst having previously been deactivated through previous use in other hydrorefining applications.
U.S. Pat. No. 4,334,982 claims the use of non-acidic supports, such as aluminates of metals such as cobalt, nickel, barium, magnesium or calcium, preferably calcium aluminate, besides specific ratios of Group VI-B and Group VIII metals.
U.S. Pat. No. 6,126,814 employs catalysts having lower metal contents (from 1 to 10 mass % MoO3 and from 0.1 to 5 mass % CoO), this hindering the stacking of MoS2 crystallites in the sulfided catalyst so as to render the catalyst more selective.
U.S. Pat. No. 5,853,570 also teaches that the metal content should be lower or the same to that required for depositing a monolayer of the metals on the support, so as to hinder crystallite stacking that favor olefin hydrogenation.
U.S. Pat. No. 2,793,170 teaches that lower pressures favor lower olefin hydrogenation degree, while hydrodesulfurization reactions are not hindered at the same degree. This document cites further that, besides the organosulfur compounds conversion reactions, a recombination reaction of the H2S reaction product also occurs with the remaining olefins, yielding mercaptan-related products. Such reactions render difficult to obtain sufficiently low sulfur contents in the product without promoting at the same time significant olefin hydrogenation. High temperatures also hinder the recombination reaction of olefins with H2S. Brazilian Application PI BR 0202413-6 (corresponding to US Application 2004/0000507) of the Applicant and herein entirely incorporated as reference, teaches the mixture of non-reactive compounds to hydrogen in order to promote he selective hydrodesulfurization reaction of a cracked olefin stream feed. The mixture promotes the dilution of hydrogen in the reaction and minimizes olefin hydrogenation without significantly reducing the organosulfur compound conversion, while aiding in the minimization of the recombination reaction by reducing the concentration of H2S generated in the reaction. There is also observed that a higher ratio of gas volume per feed volume means lower sulfur content in the product.
As regards the several non-reactive compounds, it is observed that the desired selectivity increase effect is observed not only for nitrogen, but also for the several diluent compounds and mixtures of same. It is also observed that reduced overall pressure does not lead to the same reaction selectivity as that obtained from non-reactive compounds, reducing olefin conversion but resulting also in the sulfur content increase of the product.
International publication WO 03/085068 teaches a selective hydrodesulfurization process in which a mixed feed of naphtha streams containing higher than 5 mass % olefins reacts under usual hydrodesulfurization conditions upon contact with a selective catalyst. The process aims at reducing more than 90% of the sulfur content and hydrogenating less than 60% of the feed olefins, the expected octane rating loss being higher for separately treated streams than that obtained from naphtha streams treated in admixture. The co-processing of a mixture of an olefinic naphtha stream with an effective amount, between 10% and 80 mass % of non-olefinic naphtha aims at a gain of at least 0.1 in the octane rating of the final product as compared to the separated processing of the two feeds. No other component, besides non-olefinic naphtha is considered for admixture with the olefinic naphtha. Since naphtha streams usually have similar distillation ranges, the non-olefinic naphtha will be integrated to the final gasoline pool, this limiting the application of the co-processing technique in this case.
U.S. Pat. Nos. 6,429,170 and 6,482,314 disclose a process for removing sulfur from catalytic cracking naphtha streams in a single reaction stage. The process uses a partially sulfided Ni- or Co-based regenerable reactive adsorbent on a ZnO support. The zinc oxide absorbs the H2S resulting from conversion of the organosulfurized compounds, preventing the recombination reaction, thereby resulting in process selectivity. U.S. Patent Application 2003/0232723 uses nitrogen in the adsorption process with a regenerable reactive adsorbent to boost selectivity, wherein the hydrogen molar fraction in the mixture (H2+N2) must be greater than 0.8.
In addition to the single-stage processes described above, and also in order to suppress the recombination reactions, hydrodesulfurization processes have been applied to more than one reaction stage, in which the H2S generated in the reaction is removed between the stages.
U.S. Pat. No. 2,061,845 discloses the use of more than one reaction stage with H2S removed between the stages in the hydrotreatment of cracked gasoline, leading to lesser hydrogenation of olefins and lower octane rating decrease in comparison to single-stage hydrotreatment process. U.S. Pat. No. 3,732,155 discloses the use of two stages with H2S removed between them and without the charge contacting hydrogen in the second reaction stage.
U.S. Pat. No. 3,349,027 discloses two-stage hydrotreatment of olefinic naphtha streams, with intermediate H2S removal and with a high space velocity (LHSV), making it possible to remove virtually all mercaptans. Results suggest that the mercaptan reaction rate is rather high, quickly achieving a balance between olefins present and H2S in the product.
U.S. Pat. No. 5,906,730 discloses a two-stage hydrodesulfurization process for cracked naphtha, with 60-90% of the sulfur in the charge of each stage removed, allowing for total removal of up to 99% of the sulfur in the original naphtha and with less conversion of olefins in comparison to just one reaction stage. H2S generated in each reaction step is removed before the subsequent stage, so as to hinder the formation of mercaptans resulting from the recombination of H2S with the remaining olefins. U.S. Pat. No. 5,906,730 claims the operation of the reaction stages at specific hydrogen partial pressure ranges, from 0.5 to 3.0 MPag in the first stage and 0.5 to 1.5 MPag in the second stage. The claimed hydrogen partial pressures conditions are reached for total pressure conditions and hydrogen flow rates typical for cracked naphtha HDS. This patent does not contemplate or suggest the addition of non-reactive compounds added to the reaction aiming at reducing olefin hydrogenation.
U.S. Pat. No. 6,231,753 discloses a two-stage hydrodesulfurization process, with more than 70% of the sulfur removed in the first stage and 80% of the remaining sulfur removed in the second stage, leading to a total removal of more than 95% of the sulfur so as to retain the olefins. Between the two reaction stages the generated H2S is removed. In order to obtain better selectivity (olefin preservation) as compared to previously disclosed two-stage processes, it can be seen that the temperature and LHSV in the second reactor are higher than those in the first: a temperature of 10° C. or higher, and LHSV at least 1.5 times higher.
U.S. Pat. No. 6,231,753 citing the state-of-the-art teaches that the hydrorefining units preferably recycle the non-consumed hydrogen and make up the consumed hydrogen. This patent also teaches that the composition of the hydrogen make-up streams are higher than 60% by volume, preferably higher than 80% by volume, the remaining components being inert materials such as N2, methane and the like.
The so-called cited inert materials possibly present in the make-up hydrogen originate from H2 preparation methods. The presence and concentration of the so-called inert materials depend on the presence or not and on the efficiency of the units designed for the purification of the obtained H2. Typically hydrogen is produced in units such as steam reform, or as a by-product of naphtha catalytic reform. Previously to purification processes, the hydrogen stream from the catalytic reform contains methane and light hydrocarbons, while that from the natural gas steam reform can contain N2, the presence of N2 being possible in the natural gas reform feed itself, in amounts typically lower than 10% by volume. Processes usually employed in the purification of these streams are absorption, membrane separation and molecular sieve adsorption—PSA (Pressure Swing Adsorption), among others. So-called inert compounds are considered according to state-of-the-art concepts as undesired contaminants, high-purity make-up hydrogen being employed so as to avoid inert build up in the hydrorefining unit gas recycle.
U.S. Pat. No. 6,231,753 does not consider the addition of non-reactive compounds added as a mean of minimizing olefin hydrogenation, and teaches that the hydrogen make-up stream is preferably of high purity. The amount of inert compounds present in the reaction medium, in case make-up hydrogen contains inert compounds, will depend on recycle flow rate in the system, on hydrogen consumption, on make-up flow rate, on the balance in the separator vessels and on the presence or not of a further treatment of the recycle gas for H2S removal, which can also remove a portion of the inert compounds.
U.S. patent Application 2003/0217951 discloses two reaction stages with intermediate H2S removal. This process differs from those in the previously cited patents in that more than 90% of the sulfur is converted in the first stage and the reaction rate in the second stage is slower than that in the first stage. A slower reaction rate can be obtained at a temperature lower than that in the first stage.
U.S. Pat. No. 6,736,962 discloses a two-stage process for removing sulfur, with an intermediate H2S removal step between them. A previously hydrodesulfurized olefinic naphtha, containing less than 30 mg/kg of non-mercaptidic sulfur compounds, is processed while contacting a catalyst together with a purge gas, under two possible conditions. When the purge gas is hydrogen, the second-stage catalyst is an irreducible oxide (merely a support, with no hydrogenating activity). When the purge gas is a gas compound, such as He, N2, Ar, CH4, natural gas, light gas, and mixtures of the same containing no hydrogen, the second-stage catalyst is a metal oxide of Group VIIIB promoted by a metal oxide of the supported Group VIB (hydrorefining catalyst). The invention does not contemplate mixtures of a purge gas and hydrogen.
Typical conditions for each reaction stage in HDS processes are: pressures ranging from 0.5 to 4.0 MPag, preferably from 2.0 to 3.0 MPag; temperatures ranging from 200 to 400° C., preferably from 260 to 340° C.; space velocity (volume processed per hour per volume of catalyst), or LHSV, from 1 to 10 h−1; rate of hydrogen volume per processed charge volume ranging from 35 to 720 Nm3/m3; and hydrogen purity normally higher than 80%, and preferably higher than 90%.
Literature also indicates that when H2S is removed between reaction stages, H2S concentration at the second stage intake should preferably be less than 0.05% by volume (500 ppmv), or more preferably, the H2S concentration in the gas produced by the second reactor should be less than 0.05% by volume so that it may be recycled back to the first reactor untreated.
Brazilian Application PI BR 0502040-9 of the Applicant and herein completely incorporated as reference teaches a selective hydrodesulfurization process of olefinic naphtha streams where the said process comprises two reaction stages where the feed contacts hydrogen and at least one non-reactive added compound. The generated H2S is removed so that the concentration of same at the reactor inlet does not favor the recombination to mercaptans. It could be observed that the use of added non-reactive compound in both stages resulted in higher selectivity than in state-of-the-art processes, where two reaction stages were practiced with non-reactive added compound. Unexpectedly, the use of a non-reactive compound only in the second stage resulted in still higher selectivity than in the two stages addition. However, this publication considers the use of the same catalyst in both reaction stages.
U.S. Pat. No. 6,692,635 teaches a two-stage selective hydrodesulfurization process for olefinic naphtha streams with distinct catalysts in each stage. The first stage catalyst contains Group VI-B (preferably Mo or W) and Group VIII (preferably Co or Ni) metals supported on alumina or silica-alumina or still other porous solids such as magnesia, silica or titanium oxide, as such or admixed with alumina or silica-alumina, aiming at hydrogenating thiophenic compounds to more easily desulfurizable compounds as well as removing a portion of the sulfur compounds. The second catalyst aims at decomposing the sulfur compounds and is selected among the group of Ni, Co, Fe, Mo or W, it being important to control the sulfiding degree of the catalyst. The sulfiding degree of alumina-supported Ni, as taught by U.S. Pat. No. 2,273,297 alters the reaction selectivity by more or less favoring hydrogenation to the disadvantage of desulfurization, it being possible to keep a significant desulfurization activity at a lower hydrogenation activity level.
The reactive adsorption patents, U.S. Pat. No. 6,429,170 and U.S. Pat. No. 6,482,314 cited above also make use of the nature of the nickel sulfiding degree for diminishing the hydrogenating activity.
U.S. patent Application US2004/0026298 also teaches a cracked naphtha hydrodesulfurization process in a multiple bed, where the metal content of the second bed catalyst is from 10 to 95% lower than the first bed catalyst. Both are Group VIII and Group VI-B catalysts, preferably supported on alumina, and can still have from 1.0 to 3.0 mass % of additives deposited as alkaline metals or alkaline metal oxides or phosphorus.
Multiple processes are also seen in the art, indicative of the importance and the difficulties inherent to selective processes for removing sulfur from olefinic naphtha streams.
Accordingly, there is still a need for a catalytic hydrodesulfurization process capable of reducing the sulfur content in FCC naphtha charges to the maximum, with minimum olefin hydrogenation. This objective is reached through the process comprising two reaction stages where the feed contacts a hydrogen stream and at least a non-reactive compound preferably added in the second reaction stage and is removed the H2S effluent from the first reaction stage, a more active HDS catalyst in a first reaction stage and a less active HDS catalyst in a final reaction stage being employed, such process being described and claimed in the present application.